Consider new framework for cheatsheet PDF conversion

[sjspielman]

This topic was somewhat discussed in #663, but let's formalize here.
We'd like to move to a non-interactive way to do md -> pdf that actually looks nice, without dealing with excessive/any LaTeX templates.
This npm library has a pretty nice default style! https://github.com/BlueHatbRit/mdpdf
To get a sense, here's a PDF conversion of the current version of the scAdvanced cheatsheet, simply from running the following after installing per package README.

mdpdf scRNA-seq-advanced-cheatsheet.md 

scRNA-seq-advanced-cheatsheet.pdf.

[sjspielman]

Let's get some nicer PDF conversions going for the next round of scIntro.

[sjspielman]

At this point we have Node.js libraries up and running for cheatsheets. In the future we may want to establish a GHA to handle conversions, but for now we're all set here. If we want to go the GHA route in the future it would merit a new issue.